On 14 August 2005, Lyon and Chelsea agreed a £24.4 million fee for Essien, ending one of the longest-running transfer sagas. At the time, the fee made Essien Chelsea's most expensive signing of all time, surpassing the £24 million Chelsea paid for Didier Drogba the previous summer. The transfer went through on 19 August.
Essien made his debut as a second-half sub against Arsenal on 21 August 2005, wearing the number 5 on his shirt. The match ended in a 1–0 victory to Chelsea. He made an assist in his full debut against WBA,replascing the injured Claude Makélélé in the defensive midfield role against Sunderland in the 2–0 win on 10 September 2005. He soon cemented his place in Jose Mourinho's side, starting in 31 domestic league matches, as well as 11 appearances in other cup competitions.
On 15 December 2005, Essien was given a two-match ban by UEFA for a controversial tackle on Dietmar Hamann, which resulted in his suspension from Chelsea's Champions League second round tie with eventual European champions Barcelona. Essien apologised to Hamann for the challenge, stating that while he desires to be perceived as a player who "unsettles" opponents, he was not a malicious or an unsporting player; Hamann publicly accepted Essien's apology. In January 2006,
Essien scored his first goal for Chelsea from a low cross from Shaun Wright-Phillips on 11 March 2006, in a 2–1 victory over Tottenham Hotspur. His second goal came on 17 April 2006, against Everton with a powerful strike after an assertive burst through the defensive line. Essien ended his maiden season in England with two goals, both in the Premier League.
Essien was nominated for the 2006 FIFA World Player of the Year Award on 12 October 2006, where he placed 22nd. One week later, he was nominated for the 2006 Ballon d'Or. He was voted as the third Best African Footballer of the Year in 2006, a feat he also achieved in 2005. He won the 2006 BBC African footballer of the year award.
On 15 May 2007, Chelsea fans voted Essien as Chelsea Player of the Year for his contributions in the 2006–07 season, becoming the first African to receive the honour. His late dramatic equalising goal against Arsenal was also voted as Chelsea Goal of the Season for 2006–07.
On 12 August 2007, Essien scored the winner with his first goal of the 2007-8 Premier League campaign, as Chelsea defeated Birmingham City to set an English record for unbeaten league matches at home, eclipsing Liverpool's previous top-flight record of 63 sets between 1978 and 1981.
On 10 October 2007, Essien was nominated for the prestigious 2007 FIFA World Player of the Year Award for the third consecutive year and on 21 October 2007, he was also nominated for the 2007 Ballon d'Or Award for the third consecutive year. He was voted 15th Best Player in the World at the 2007 FIFA World Player of the Year Awards. On 12 December 2007, he was nominated for the 2007 African Footballer of the Year, the third consecutive time he had been nominated. He was runner-up to that year's winner, Frederic Kanoute.
On 22 July 2008, Essien signed a new five-year deal with Chelsea, keeping him at the club until 2013. He also extended his contract on 12 March 2007.
It was feared Essien would miss the 2008-9 Premier season after sustaining acl damage on 5 September while playing for Ghana. However, on 7 March 2009, he came on as a substitute in the second half of the FA Cup match against Coventry City. In his second match back from the knee injury, Essien scored just before half-time to give Chelsea a vital away goal against Juventus in the Champions League first knockout round. Chelsea went on the draw the match 2–2 and through to the quarter-finals 3–2 on aggregate. He made his first league start from injury against Manchester City, scoring the only goal in the match.
On 6 May 2009, Essien scored a left-footed volley in the second leg of the Champions League semifinal against Barcelona, a tie that Chelsea ultimately lost due to the away goal rule. The goal was later voted as Chelsea's Goal of the Season by the club's fans.
In the 2009–10 Premier League season, Essien put in an excellent performance against Blackburn Rovers on 24 October 2009 and scored his first goal of the season with a swerving 35-yard shot on 52 minutes in a 5–0 victory at Stamford Bridge. He scored his first brace for Chelsea against Wolverhampton Wanderers with a header and a low shot from outside the box and came close to completing his first hat-trick for the club with a flurry of second-half shots, one of which was tipped onto the bar by Wolves goalkeeper Wayne Hennessey. Essien was injured on international duty during the 2010 African Nations Cup and missed the remainder of the English domestic season. On 4 June 2010, he signed a two-year extension to his existing contract which would have kept him at the club until 2015. Essien returned to action on 17 July 2010 in a friendly against Crystal Palace and scored the only goal of the match to give Chelsea the win.
After missing the 2010 FIFA World Cup through injury, Essien had a bright start to the season for Chelsea. He scored the only goal in a pre-season friendly win against Crystal Palace and was an ever-present in Chelsea's unbeaten month of August. He scored a brace against West Ham United, and he scored against MSK Zillina in the UEFA Champions League. Essien also put in impressive performances against Marseille, Blackburn Rovers and Fulham, the latter a match in which he scored the only goal and received a red card. However, Essien would again incur an injury and missed two months of the season, during which Chelsea won just one out of nine matches. He returned to score in the penalty shoot-out loss in the FA Cup against Everton but ultimately had a poor second half of the season. After Chelsea's final match, the Ghana FA announced Essien had returned to the international squad.
In pre-season training for the 2011–12 Premier League season, Essien ruptured his anterior cruciate ligament and meniscus, undergoing surgery on 11 July 2011. His recovery was said to take up to six months. However, he began training lightly in late November 2011 and made his comeback on 9 January 2012, playing 75 minutes of the reserves' 3–2 defeat to West Bromwich Albion. He made his 150th Premier League appearance for Chelsea as a substitute in a 1–0 win over Sunderland. On 21 January 2012, Essien featured as late substitute in Chelsea's 0–0 draw away at Norwich City.
Due to missing most of the beginning half of the 2011–12 Premier League season, Essien was unable to gain favour with manager André Villas-Boas. On 21 February 2012, during a Champions' League match against Napoli, Villas-Boas left Frank Lampard, Essien and Ashley Cole on the substitutes' bench. Chelsea lost 3–1 and the club's technical director asked for an explanation of the team selection on behalf of Chelsea owner Abramovic!! On 4 March 2012, following a 1–0 league defeat against West Brom which left Chelsea three points adrift of Arsenal in the battle for fourth place in the Premier League, Villas-Boas' employment was terminated by Chelsea, with assistant manager Robert Di Matteo being appointed as caretaker manager on an interim basis until the end of the season. Di Matteo immediately showed favour towards veteran Chelsea players like Essien, Lampard and John Terry. Subsequently, Essien started in Chelsea's second leg champions league fixture against Napoli, with Chelsea winning 4–1 – and 5–4 on aggregate – in an entertaining match allowing Chelsea to advance to the quarter-final, where they met Benfica. Essien subsequently remained an unused substitute in Chelsea's Champions League run, culminating in The Blues' victory over Bayern Munich in the Final making Chelsea the first London-based club to win the title. Cheer Up Manager!!
